Hypothesis, that CHO availability plays a key role in the regulation of N balance by sequestering and transporting free N from amino acid and purine nucleotide catabolism and continuously replenishing TCA cycle intermediates, thereby precluding the need for amino carbon in these roles. Stable isotopes are used to determine the effect of carbohydrate availability on protein and amino acid metabolism in normal volunteers at rest, during exercise, and recovery while glycogen is either surfeit or depleted.